PROJECT SUMMARY. This transformative R01 project will test the paradigm-shifting hypothesis that Down syndrome (DS) could be understood, in large measure, as an immune disorder driven by hyperactive Interferon (IFN) signaling. This hypothesis is supported by a large body of evidence demonstrating strong and widespread immune dysregulation across the lifespan of people with DS. More specifically, this project arises from the recent discovery by this team showing that trisomy 21 (T21) causes consistent activation of the IFN response in multiple cell types, concurrent with proteomics and metabolomics changes indicative of chronic IFN hyperactivity and autoinflammation in living individuals with DS. Remarkably, these results could be explained simply by the fact that four of the six IFN receptors (IFNRs) are encoded on chromosome 21. Furthermore, the rationale for this project is strongly supported by a large body of literature demonstrating the negative impacts of dysregulated IFN signaling on human health in the general population, including key roles in the etiology of diseases that are more prevalent in people with T21. To define the role of IFN hyperactivity in DS, this team will implement a synergistic combination of human research, employing an ongoing pan-omics cohort study of the population with DS, and animal research in advanced mouse models of DS. Human research efforts will define associations between markers of IFN hyperactivity, immune dysregulation, and DS co-morbidities in 300 individuals with DS, creating a rich dataset that would also enable the testing of myriad alternative hypotheses by this team and others in the field. Animal research will use mouse models of DS along with a novel mouse strain lacking the IFNR gene cluster to determine cause-effect relationships between increased IFNR gene dosage, hyperactive IFN signaling, and development of common DS co-morbidities. The transformational nature of this project cannot be overstated. If the observed chronic activation of the IFN response and consequent immune dysregulation are indeed driving many of the developmental and clinical hallmarks of DS, research on the biology of DS would have to be redirected toward the field of immunology. If the IFNR gene cluster on chromosome 21 is indeed a major contributor to DS phenotypes and co-morbidities, our clinical understanding of DS would be reframed, justifying the development of immune-based diagnostic and therapeutic strategies to improve health outcomes in people with T21. Ultimately, this research project could not only change the way we study DS in the laboratory and how DS is managed in the clinic, but would also advance our mechanistic understanding of the role of IFN signaling and the immune system in the myriad medical conditions that are modulated, either positively or negatively, by T21, thus advancing the NIH mission on multiple fronts.